Smoothing method for layered deposition modeling

ABSTRACT

Disclosed is a method for smoothing the surface of an object built from a polymeric or wax material using a layered manufacturing rapid prototyping technique. After the object is built it is exposed to a vaporized solvent such as in a vaporizer for an exposure time sufficient to reflow the object surface. A solvent is chosen based on its ability to transiently soften the material which forms the object, and thereafter evaporate off the object. The object is removed from the solvent and allowed to dry, producing a smooth finished part.

BACKGROUND OF THE INVENTION

The present invention relates to the field of rapid prototyping, andparticularly to methods of achieving surface smoothness in prototypeobjects made by layered manufacturing.

Production and testing of prototype objects is a commonly used step indeveloping new products, machines and processes in a wide range ofindustries. A variety of layered manufacturing methods for formingthree-dimensional prototypes are known, which develop prototype objectscheaply and quickly from a geometric computer model under computercontrol. These rapid prototyping methods generally slice or divide adigital representation of a desired object (computer aided design (CAD))into horizontal layers, then build the object layer-by-layer byrepetitive application of materials. Exemplary rapid prototypingtechniques include layered deposition modeling, selective lasersintering and stereolithographic processes.

One example of layered deposition modeling is a fused depositionmodeling technique performed by Stratasys® FDM® modeling machines. Fuseddeposition modeling builds up three-dimensional objects by extrudingsolidifiable modeling material from an extrusion head in a predeterminedpattern, layer-by-layer, based upon design data corresponding to theparticular shape of each object layer. Examples of extrusion-basedapparatus and methods for making three-dimensional objects are describedin Crump U.S. Pat. No. 5,121,329, Crump U.S. Pat. No. 5,340,433,Danforth et al. U.S. Pat. No. 5,738,817, Batchelder et al. U.S. Pat. No.5,764,521 and Dahlin et al. U.S. Pat. No. 6,022,207, all of which areassigned to Stratasys, Inc., the assignee of the present invention.

In the Stratasys® FDM® modeling machines of the current art, modelingmaterial is typically loaded into the machine as a flexible filamentwound on a supply reel, such as disclosed in U.S. Pat. No. 5,121,329. Asolidifiable material which adheres to the previous layer with anadequate bond upon solidification and which can be supplied as aflexible filament is used as the modeling material Motor-driven feedrollers advance the strand of filament into a liquifier carried by anextrusion head. Inside the liquifier, the filament is heated to aflowable temperature. Flowable modeling material is forced out of anozzle on the far end of the liquifier, and deposited from the liquifieronto a base. The flow rate of the material extruded from the nozzle is afunction of the rate at which the filament is advanced to the extrusionhead. A controller controls movement of the extrusion head in ahorizontal x, y plane, controls movement of the base in a verticalz-direction, and controls the rate at which the feed rollers advancefilament. By controlling these processing variables in synchrony, themodeling material is deposited in “beads” layer-by-layer along toolpaths defined from the CAD model. The material being extruded fuses topreviously deposited material and solidifies to form a three-dimensionalobject resembling the CAD model.

The surfaces of objects developed from layered manufacturing techniquesof the current art are textured or striated due to their layeredformation. Curved and angled surfaces generally have a “stair step”appearance, caused by layering of cross-sectional shapes which havesquare edge profiles. The stair-stepping effect is more pronounced aslayer thickness increases. Although the stair-stepping does not effectthe strength of the object, it does detract aesthetically.

Surface roughness of objects created by layered manufacturing techniquesalso arises from errors in the build process. For example, in the fuseddeposition modeling systems of the current art, errors can arise due inpart to inconsistent extrusion flow rates. Errors particularly occur atstart points and end points of the tool path, for instance, at thelocation of a “seam” (i.e., the start and end point of a closed-looptool path). These errors can cause undesired inconsistencies in theshape of the resulting model.

Rapid prototyping of three-dimensional objects includes not only theproduction of prototype parts, but also may include the production ofmolds. Exemplary uses of molds created with rapid prototyping includeforming molds used to create injection molding tool inserts such asdescribed in U.S. Pat. No. 5,189,781, and forming fugitive molds forgreen ceramic pieces prior to sintering such as described in U.S. Pat.No. 5,824,250 and U.S. Pat. No. 5,976,457.

The current art teaches manually trimming, machining or grinding objectsformed by layered manufacturing to remove excess material. Buffing withcloths, sand paper or solution-born abrasives are current methods ofsmoothing or polishing the objects. For example, Hull et al. U.S. Pat.No. 5,059,359, Methods and Apparatus for Producing Three-dimensionalObjects by Stereolithography, describes their prototypes as “perfect forsmoothing by sanding to yield the right-sized part”. The need forhand-finishing of models made from additive process techniques is alsorecognized in U.S. Pat. No. 6,021,358, which utilizes subtractivemodeling techniques to attain smooth models. There is a need in rapidprototyping systems of an expedient and relatively inexpensive method ofpost-processing layered manufacturing prototype objects.

A previously developed technique used in manufacturing of plasticsinvolves the use of chemical vapors or liquids to smooth by reflowingthe surface of the plastic, termed solvent polishing. Solvent polishingwas developed in the plastics industry over twenty years ago for thepurpose of developing a smooth level and/or high gloss coating orsurface without needing to exercise extreme care in the application ormanufacturing of the items. For example, U.S. Pat. No. 3,437,727discloses a method using chemical vapors for refinishing telephones thatwere returned to the telephone company as a method of recycling them.

There are two standard methods for solvent polishing articles. The firstis to immerse the entire plastic article in a bath of liquid plasticsolvent for a period of time based on the solvent and type of plasticinvolved. This allows the solvent to penetrate the outer layer of theplastic, thereby causing it to reflow. Reflowing causes the outersurfaces of the plastic article to become smooth and/or shiny.

The second method of solvent polishing is the exposure of the plasticarticle to vaporized solvent. A handheld vaporizer as shown in U.S. Pat.No. 4,260,873 may be used to expose the plastic object. Alternatively,the object can be placed into a chamber filled with a vaporized solvent,generated from a heated bath below, as in U.S. Pat. No. 3,737,499. Anadvantage of the vaporizing chamber is that the solvent is contained andcan be recycled, thereby minimizing potential environmental pollution.

The use of hot solvent vapors to melt or plasticize the surface of thesubstrate has been used in the circuit board manufacturing area tofacilitate the transfer of printed circuits, as disclosed, for example,in U.S. Pat. No. 4,976,813. Another example is disclosed in U.S. Pat.No. 4,594,311, where solvent vapor is used to treat the non-imaged areasof the plastic base material which holds a printed circuit board inorder to further enhance the printed pattern and secure it more stronglyto the surface. In U.S. Pat. No. 5,045,141, a substrate, typically acircuit board, may be treated to facilitate transfer of the printedcircuit to it.

Solvent polishing using liquid or vapors is also commonly used as adegreasing or cleaning step in manufacturing processes, especially priorto painting.

Despite the need in rapid prototyping for an expedient and inexpensivesurface finishing technique, Applicant is unaware of any teaching orsuggestion in the prior art to use a vapor polishing technique for thesmoothing of objects formed by layered manufacturing rapid prototypingtechniques.

BRIEF SUMMARY OF THE INVENTION

The present invention is a method for smoothing the surface of an objectbuilt from a polymeric or wax material using a layered manufacturingrapid prototyping technique. After the object is built, it is exposed toa vaporized solvent for an exposure time sufficient to reflow the objectsurface. A solvent is chosen based on its ability to transiently softenthe material which forms the object, and thereafter evaporate off theobject. The object is removed from the solvent and allowed to dry,producing a smooth finished part. Optionally, portions of the objectsurface may be masked prior to exposing the object to solvent, so as topreserve fine details of the object. Alternatively, portions of theobject surface may be pre-distorted prior to exposing the object tosolvent, so that said surface portions will attain a desired geometryfollowing vapor smoothing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, magnified view illustrating a raw object formedby a layered manufacturing rapid prototyping technique.

FIG. 2 is a perspective, magnified view of the object shown in FIG. 1,after undergoing vapor smoothing.

FIG. 3 is a diagrammatic view illustrating the process of vaporsmoothing an object in accordance with the present invention.

FIG. 4 a is a cross-sectional detailed view of a portion of a raw objectformed by fused deposition modeling.

FIG. 4 b shows the object cross-section of FIG. 4 a after vaporsmoothing.

FIG. 5 a is a cross-sectional detailed view of the object portion shownin FIG. 4 a, wherein the object geometry has been pre-distorted inanticipation of vapor smoothing.

FIG. 5 b shows the object cross-section of FIG. 5 a after vaporsmoothing.

DETAILED DESCRIPTION

The method of the present invention may be employed with respect toobjects formed from a polymeric or wax material using layeredmanufacturing rapid prototyping techniques. An exemplary layeredmanufacturing technique is the type disclosed in U.S. Pat. No.5,121,329, wherein an extrusion head deposits “roads” of molten materialin layers of predetermined shape, and which material solidifies upon adrop in temperature to form a solid model.

FIG. 1 shows an exemplary object 10, formed by a layered manufacturingrapid prototyping technique. The object 10 has an angled surface 12, acurved surface 14, two horizontal surfaces 16, and three verticalsurfaces 18. In another embodiment, the object may be a mold tool foruse in making prototype plastic injection molded parts, such as isdisclosed in International Application No. PCT/US03/______ entitled“Layered Deposition Bridge Tooling”, S. Crump and J. Hanson, filed oneven date herewith and assigned to the same assignee as the presentapplication. The object 10 is made of a polymeric or wax modelingmaterial, which may also include fillers and other additives as well asdispersed particulate materials. Amorphous thermoplastics areparticularly suited for use in the present invention, for instance, ABS,polycarbonate, polyphenylsulfone, polysulfone, polystyrene,polyphenylene ether, amorphous polyamides, acrylics,poly(2-ethyl-2-oxazoline), and blends thereof. The present invention mayalso be used to advantage with other polymeric and wax materials,including glass-filled nylon, jetting wax, sintered thermal plasticpowders, and green metals or green ceramics dispersed in a polymericbinder.

As shown in FIG. 1, the object 10 is “raw”, that is, it has notundergone post-process smoothing. Prior to vapor smoothing in accordancewith the present invention, surfaces 12 and 14 exhibit a stair-steppingeffect. Surfaces 16 and 18 exhibit striation and roughness.

To smooth the surfaces of object 10, the object 10 is placed in avaporizer 30, where it is exposed to vapors of a solvent 34. This isillustrated in FIG. 3. A suitable vaporizer is of the type availablefrom Detrex Corp. of Southfield, Mich., model VS-2000, although thoseskilled in the art will recognize that many alternative vaporizers canbe used in practicing the present invention. Vaporizer 30 is shown ashaving a control panel 31 for controlling operation of the vaporizer,and primary and secondary cooling coils, 33 and 35, respectively.

The solvent 34 is selected to be compatible with the modeling materialwhich forms the object 10. Suitable solvents will react with themodeling material so as to soften and flow the material at the objectsurfaces. The solvent should also be able to vaporize off the surface ofthe object, leaving the object intact and unscathed. A preferred solventfor use with a wide range of amorphous thermoplastics is methylenechloride. Other suitable solvents will be recognized by those skilled inthe art, for instance, an n-Propyl bromide solution (e.g., Abzol®),perchloroethylene, trichloroethylene, and a hydrofluorocarbon fluid soldunder the name Vertrel®.

As illustrated in FIG. 3, the vaporizer 30 boils the solvent 34 into avapor zone 36, which is maintained at or above the boiling point of thesolvent and contained by the cooling coils 33 and 35. The object 10 issuspended in the vapor zone 36, held by a wire skewer 32, which is bentto fit around the object. Alternative holding means may also be used,such as a basket, a net or a mesh platform. The object 10 is exposed tothe vaporized solvent 34, allowing vapors of the solvent 34 to penetratethe surfaces 12, 14, 16 and 18 of object 10. Penetration of the solvent34 softens the modeling material at the object surfaces, so that thesurface material may reflow. Reflowing of the material smooths theobject surfaces.

The object 10 remains exposed to the vapors of solvent 34 until adesired surface finish is obtained. An exposure time is selected basedon the type of solvent and modeling material, the fineness of the objectfeatures, and the concentration of the solvent vapors. The exposure timecan be gauged by observing condensation of solvent vapors on the object,or can be pre-selected according to a formula. Condensation will stopwhen the temperature of the object surface reaches the temperature ofthe boiling solvent. This is an indication that solvent penetration hasoccurred. Using methylene chloride as the solvent 34, an amorphousthermoplastic modeling material will soften and reflow in a short time,typically between about 0.1 seconds to 5 minutes exposure time. Ifsmoothing of an object is expected to occur in a short exposure time, itmay be desirable to reduce the concentration of solvent vapors so thatthe exposure time can be increased. A longer exposure time allows anoperator more time to observe the smoothing process and more room forerror in removing the object from the solvent vapors.

When the exposure time elapses, the object 10 is removed from the vaporzone 36 and allowed to dry. In a preferred embodiment, the object 10dries within seconds to minutes after its removal from the vapor zone36, as the solvent 34 evaporates off of the object 10. The object 10 maythen be handled, as it is not sticky, soft or wet. In some cases, suchas where solvent exposure time is great or the solvent is highlyconcentrated, it may be desirable to dry the object 10 in an oven toremove any residual solvent. Oven drying should be done at a temperaturegreater than the boiling point of the solvent.

Following the vapor smoothing process, the stair steps in surfaces 12,14, 16 and 18 of object 10 are either significantly reduced oreliminated. The extent of the smoothing achieved for a given objectusing the method of the present invention will depending upon theexposure time, the solvent, the modeling material, and the initialsurface condition of the object. FIG. 2 illustrates a significantreduction in the stair steps and roughness of the object 10, achieved byvapor smoothing.

Optionally, selected features of an object (e.g., features smaller than0.25 inches, thin walls, corners, convex edges and concave edges) can bemasked with a substance that will inhibit smoothing of said selectedportions, or, exposure of said selected features to the solvent vaporscan be otherwise avoided. For example, it may be desirable to mask thecorners of object 10, to prevent the corners from rounding. Similarly,concave surfaces of an object can be masked to prevent in-flow ofsurrounding material. Suitable solvent masking substances include thoseused in printed circuit board manufacturing, such as gums, waxes,pastes, adhesives or masking tape, which may be applied either manuallyor automatically. Masking may also be accomplished by surrounding afeature with a gas.

Automatic application of a masking substrate may be done, for example,by jetting a masking material onto the surface of selected objectfeatures, in a layered deposition process such as is known in the art. Amasking substance may also be applied by depositing roads of maskingmaterial, using a fused deposition modeling process such as performed byStratasys® FDM® modeling machines. Those skilled in the art willrecognize additional masking techniques know in the art, that may beapplied in carrying out the present invention.

When an automatic masking technique is used, the features to be maskedmay be identified using a software algorithm that creates a digitalrepresentation of the surface area to be protected. The protected areamay be identified in a digital representation of the object, such as inan .stl file geometry using a CAD system, a Graphic Pixel system or aVoxel system. Alternatively, the surface areas to be masked may beidentified by the user via a haptic input interface, such as a FreeForm™system available from SensAble Technologies, Inc. The haptic inputsystem creates a digital mask of the areas for which smoothing is notdesired.

As an alternative to masking techniques, the geometry of an objectsurface may be pre-distorted in anticipation of the vapor smoothing. Thepre-distortion is accomplished by using a software algorithm to modify adigital representation of the object (e.g., a CAD model of the object ora sliced representation of the object as in a .stl file). Using apre-distortion software algorithm, feature details are distorted so asto overbuild corners and edges, and underbuild pockets, such thatfollowing vapor smoothing such features will attain approximately thedesired geometry. More specifically, an exemplary pre-distortionalgorithm will: (1) identify geometric features with radii of curvatureequal to or smaller than the slice height (i.e., the thickness of alayer); (2) for identified features having a positive radius ofcurvature (e.g., a corner or edge), the algorithm will build up theinitial object representation at such features; and (3) for identifiedgeometric features having a negative radius of curvature (e.g., a pocketor a joint between planes), the algorithm will hollow out the objectrepresentation in the vicinity of such features. The pre-distortionsoftware algorithm thus creates a modified object representation, sothat the identified geometric features will be distorted by eitherdepositing additional material or depositing less material than isultimately desired in the final smoothed object. A similar algorithm canbe used to identify features for masking.

According to the pre-distortion algorithm, features should be built upby not more than the slice height, for instance, by half of a sliceheight. The surface roughness of a typical part made by fused depositionmodeling is about 0.3 times the slice height. The additional materialadded in pre-distortion of positive features may be roughly thethickness of this surface roughness, so that when the reflowed materialis pulled away, the solid material left takes on the desired finalobject geometry. For the negative curvature regions, enough materialneeds to be removed by the pre-distortion algorithm that the in-flowfrom the surrounding regions fills in the removed material.

Pre-distortion of object geometry is illustrated in FIGS. 4 a and 4 band FIGS. 5 a and 5 b. FIGS. 4 a and 4 b show a cross-sectional view ofa portion of an object 40 that has not been pre-distorted, superimposedonto an outline 42 illustrating the desired final surface objectgeometry of object 40 (i.e., the unmodified object representation). Asillustrated in FIG. 4 b, vapor smoothing results in rounding of convexcorners 44 away from the desired outline 42, and rounding of edges 46beyond the desired outline 42. FIGS. 5 a and 5 b illustrate a portion ofan object 40′ which has the same desired final surface geometry asobject 40. Unlike object 40, object 40′ has been pre-distorted accordingto the pre-distortion algorithm of the present invention. As illustratedin FIG. 5 a, the pre-distorted surface geometry of object 40 extendsbeyond the desired outline 42 at corners 44 and concave edges 46.Following vapor smoothing, as illustrated in FIG. 5 b, the corners 44and edges 46 of the pre-distorted object 40 more closely follow thedesired outline 42 than do the corners 44 and edges 46 of the object 40.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A method for making a three-dimensional object comprising the stepsof: building an object from a polymeric or wax modeling material using alayered manufacturing rapid prototyping technique; and smoothing anobject surface by exposing the object to vapors of a solvent thattransiently softens the modeling material.
 2. The method of claim 1,wherein the layered manufacturing technique is fused depositionmodeling.
 3. The method of claim 1, where the modeling material is athermoplastic resin.
 4. The method of claim 3, wherein the thermoplasticresin comprises at least about 50 weight percent of an amorphousthermoplastic selected from the group consisting of ABS, polycarbonate,polyphenylsulfone, polysulfone, polystyrene, polyphenylene ether,amorphous polyamides, acrylics, poly(2-ethyl-2-oxazoline), and blendsthereof.
 5. The method of claim 4, wherein the solvent is selected fromthe group consisting of methylene chloride, an n-Propyl bromidesolution, perchloroethylene, trichloroethylene, and a hydrofluorocarbonfluid.
 6. The method of claim 1, wherein the modeling material isselected from the group consisting of thermoplastics, green metalsdispersed in a polymeric binder, green ceramics dispersed in a polymericbinder, and jetting wax.
 7. The method of claim 6, wherein the modelingmaterial is glass-filled nylon.
 8. The method of claim 1, and furthercomprising the step of: selecting a length of time during which theobject is to be exposed to the solvent vapors as a function ofconcentration of the solvent vapors, prior to the smoothing step.
 9. Themethod of claim 8, and further comprising the step of: reducing theconcentration of solvent vapors so that the selected exposure time willincrease.
 10. The method of claim 1, and further comprising the step of:masking selected portions of the object surface with a substance thatwill inhibit smoothing of the selected portions, prior to the step ofsmoothing the object surface.
 11. The method of claim 10, wherein themasking substance is applied using an automatic process.
 12. The methodof claim 11, wherein the automatic process is a jetting process.
 13. Themethod of claim 11, wherein the automatic process is a fused depositionmodeling process.
 14. The method of claim 11, and further comprising thestep of: identifying the selected portions of the object surface formasking accordingly to their geometry.
 15. The method of claim 14, andfurther comprising the step of: identifying the selected portions of theobject surface for masking accordingly to their radii of curvature. 16.The method of claim 11, and further comprising the step of: identifyingthe selected portions of the object surface using a software algorithmthat creates a digital representation of the surface area to beprotected.
 17. The method of claim 16, wherein digital data identifyingthe surface area to be protected is stored in an .stl file.
 18. Themethod of claim 1, and further comprising the step of: creating adigital mask of selected portions of the object surface for whichsmoothing is not desired, using a haptic input interface.
 19. The methodof claim 1, wherein the building step comprises pre-distorting certainobject features so that said features will obtain a desired geometryfollowing the smoothing step.
 20. The method of claim 19, and furthercomprising the steps of: providing an initial object representation in adigital format, the initial object representation having a surfacegeometry; and modifying the initial object representation to pre-distortcertain features of the surface geometry, producing a modified objectrepresentation; wherein the object built in the building step has ageometry defined according to the modified object representation; andwherein the desired geometry attained following the smoothing stepapproximately matches that of the initial object representation.
 21. Amethod for eliminating surface roughness of an object built from amodeling material using a layered manufacturing rapid prototypingtechnique, comprising the step of: reflowing a surface of the object byexposing the object to vapors of a solvent that transiently softens themodeling material.
 22. The method of claim 21, where the modelingmaterial is a thermoplastic resin.
 23. The method of claim 22, whereinthe thermoplastic resin comprises at least about 50 weight percent of anamorphous thermoplastic selected from the group consisting of ABS,polycarbonate, polyphenylsulfone, polysulfone, polystyrene,polyphenylene ether, amorphous polyamide, methyl methacrylate,poly(2-ethyl-2-oxazoline), and blends thereof.
 24. The method of claim23, wherein the solvent is selected from the group consisting ofmethylene chloride, an n-Propyl bromide solution, perchloroethylene,trichloroethylene, and a hydrofluorocarbon fluid.
 25. The method ofclaim 21, wherein the modeling material is selected from the groupconsisting of thermoplastics, green metals dispersed in a polymericbinder, green ceramics dispersed in a polymeric binder, and jetting wax.26. The method of claim 25, wherein the modeling material isglass-filled nylon.
 27. The method of claim 21, and further comprisingthe step of: masking selected portions of the object surface with asubstance that will inhibit smoothing of the selected portions, prior tothe step of reflowing the surface.
 28. The method of claim 27, whereinthe masking substance is applied using an automatic process.
 29. Themethod of claim 28, wherein the automatic process is a jetting process.30. The method of claim 28, wherein the automatic process is a fuseddeposition modeling process.
 31. The method of claim 28, and furthercomprising the step of: identifying the selected portions of the objectsurface for masking accordingly to their geometry.
 32. The method ofclaim 31, and further comprising the step of: identifying the selectedportions of the object surface for masking accordingly to their radii ofcurvature.
 33. The method of claim 28, and further comprising the stepof: identifying the selected portions of the object surface using asoftware algorithm that creates a digital representation of the surfacearea to be protected.
 34. The method of claim 33, wherein digital dataidentifying the surface area to be protected is stored in an .stl file.35. The method of claim 28, and further comprising the step of:identifying the selected portions of the object surface for maskingusing a haptic input interface.
 36. A method for making athree-dimensional object comprising the steps of: providing an initialobject representation in a digital format, the initial objectrepresentation having a surface geometry; modifying the initial objectrepresentation to pre-distort certain features of the surface geometry,producing a modified object representation; building an object asdefined by the modified object representation, from a modeling materialusing a layered manufacturing technique; and vapor smoothing surfaces ofthe object to produce a finished object, the finished object having asurface geometry that approximately matches that of the initial objectrepresentation.
 37. The method of claim 36, and further comprising thestep of: identifying features of the surface geometry for pre-distortionaccording to their radii of curvature.